1. Field of the Invention
The present invention relates to an exchanging converter with a zero potential switching control function, and more particularly to such a converter, which drives every switch to achieve a switching operation at a zero-voltage status, so as to effectively eliminate power loss due to a high frequency switching operation.
2. Description of the Prior Art
In recent years, semiconductor fabrication technology has rapidly developed, and semiconductor elements are made more and more smaller. This development trend in semiconductor fabrication drives electronic product manufacturers to create and design thinner, lighter and shorter products. However, in conventional hard type exchanging converters, a power switch is operated under a high frequency environment. This operation consumes much power, and produces much heat. In order to prevent damage due to heat, a heat sink, fan, and suitable cooling means must be installed to carry heat from the converter. Further, these conventional hard type-switching converters are expensive, and wear quickly with use. Because of the aforesaid numerous drawbacks, conventional hard type switching converters cannot be made as small as desired.
Since 1980, following the development of microcomputer, "small-size" has become more and more important in the fabrication of electronic products. In order to meet this requirement, the following converters are developed:
(1) Flyback Converter
A flyback converter, as shown in FIG. 1, comprises an input voltage filter capacitor C.sub.1 connected between two opposite ends of an input power source V.sub.in to provide a stable input voltage to a posterior converter. The posterior converter comprises a transformer. The transformer comprises a primary winding L.sub.p and a secondary winding L.sub.s. The primary winding L.sub.p is connected to a switching element S.sub.1, forming a series loop at two opposite sides of the filter capacitor C.sub.1. The secondary winding L.sub.s is connected to a diode D.sub.1, forming a series loop at two opposite sides of a filter capacitor C.sub.2. Modulated high frequency from the switching element S.sub.1 is smoothed through the posterior converter, so that the transformer provides a DC output voltage V.sub.o to the load. In this flyback converter, when the switching element S.sub.1 is on, input power source V.sub.in charges the primary winding L.sub.p, enabling energy to be stored therein. At this time, the polarity of the primary winding L.sub.p is reversed to the secondary winding L.sub.s, and the diode D.sub.1 is biased reversely, and therefore the output voltage filter capacitor C.sub.2 provides the load with the necessary energy. When the switching element S.sub.1 is off, the magnetic flux at the transformer starts to contract, and the voltage polarity of the second winding L.sub.s is reversed to produce an induced current, thereby causing the diode D.sub.1 to be electrically connected. When the diode D.sub.1 is electrically connected, it charges the filter capacitor C.sub.2, enabling electricity to be outputted to the load. Because a high voltage exists in the switching element S.sub.1 when the switching element S.sub.1 is off, a potential energy (CV.sup.2 /2) is accumulated in its parasitic capacitance. This potential energy (CV.sup.2 /2) is changed into heat energy at the moment the switching element S.sub.1 is switched off. Therefore, the switching element produces high heat under a high frequency switching environment, and wears quickly with use. U.S. Pat. No. 5,057,986 discloses a converter, which eliminates the aforesaid problem. According to this design, as shown in FIG. 2, another switching element S.sub.2 and capacitor C.sub.p are added to the primary circuit, so that the resonance formed at the parasitic capacitance of the inductance Lp, capacitor C.sub.p and switching elements S.sub.1 ;S.sub.2 of the converter is utilized to achieve a zero-voltage control scheme. However, because this zero-voltage control scheme provides the necessary energy for zero-voltage control by means of the inductor Lp, zero-voltage control becomes more difficult to achieve when the load is high. U.S. Pat. No. 5,402,329 discloses another design, in which, as shown in FIG. 3, a small inductance L.sub.1 is installed in the converter to provide the necessary energy for zero-voltage control. This inductance can be an externally added inductance, or a leakage inductance of the transformer itself. This design eliminates the drawback of the disclosure of U.S. Pat. No. 5,057,986, however because the zero-voltage control of this design relies on the stray capacitance and leakage inductance of the circuit, which is difficult to specify when designing and fabricating this structure of converter.
(2) Boost Converter
A boost converter, as shown in FIG. 4, is used to improve power factor correction. Because power factor correction runs under a high voltage environment, a voltage of about 400V exists when the switching element S.sub.1 is switched off, and accumulated high electric energy will be changed into heat energy at the switching element S.sub.1 at the moment the switching element S.sub.1 is switched on, causing the service life of the switching element S.sub.1 to be shortened. In 1992 Lee Yuan-Tse et al disclosed another design of converter, in which, as shown in FIG. 5, an auxiliary switch S.sub.2, an inductor L.sub.2 and a diode D.sub.2 are added to the circuit shown in FIG. 4. When operated, the auxiliary switch S.sub.2 is transiently turned on and maintained electrically connected until the voltage at the switching element S.sub.1 is discharged, and the switching element S.sub.1 is turned on to complete zero-voltage switching control when reached the status of zero-voltage is reached. This design greatly increases the cost. Because of high cost, this design is not popularly accepted. FIG. 6 shows another design according to U.S. Pat. No. 5,402,329. This design simply reduces discharge loss at the switching element S.sub.1 due to deposit charge at the rectifier diode D.sub.1. However, because the switching element S.sub.1 uses a hard type switching mode, power loss under a high frequency switching operation is still significant.
(3) Buck Converter
A buck converter, as shown in FIG. 7, is designed for use under a low voltage high current condition. This design biases the reduction of turn-on power loss at the switching element S.sub.1 and the rectifier diode D.sub.1, however it neglects switching loss, and no application example of this design has been disclosed. FIG. 8 shows another design of a buck converter, in which power MOSFET transistors Q.sub.1 and Q.sub.2 are used as substitutes for the switching element S.sub.1 and the rectifier diode D.sub.1. Because these two transistors Q.sub.1 and Q.sub.2 adopt complementary switching, the advantage of low impedance of these two transistors Q.sub.1 and Q.sub.2 is used to reduce turn-on loss. However, because these two transistors Q.sub.1 and Q.sub.2 require a hard switching mode, a high power loss during switching of the switching element is inevitable when used in a high voltage condition.